1.5 litres of saliva is produced by the human body every day, essential for carrying out a vital role in lubricating food, digestion, and protecting the oral environment.
The production and composition of saliva is under neural control – via the parasympathetic and sympathetic nervous systems.
In this article, we shall look at the regulation of salivary production and its clinical correlations.
Production of Saliva
Saliva is produced by the salivary glands of body – the parotid, submandibular and sublingual glands. Within the glands, the acinar cells are responsible for the volume of saliva secreted, and the duct cells are responsible for the composition of saliva.
Information on the secretion of saliva can be found here.
Saliva is produced and secreted by the salivary glands of the body. These glands are under the control of the autonomic nervous system, comprised of sympathetic and parasympathetic nerve fibres.
The sympathetic control of salivary production is via the superior cervical ganglion. Sympathetic stimulation results in the release of noradrenaline, which acts upon alpha- and beta-adrenergic receptors.
This results in the following effects:
- Decreased production of saliva by acinar cells
- Increased protein secretion
- Decreased blood flow to the glands
There is variable sympathetic innervation between the salivary glands. On the whole, this system is far less important than the parasympathetic innervation in terms of regulating the production of saliva.
The parasympathetic outflow is coordinated via centres in the medulla, and innervation occurs via the facial and glossopharyngeal nerves. Afferent information from the mouth, tongue, nose and conditioned reflexes are integrated within the brain – and in the presence of food, parasympathetic stimulation occurs.
Parasympathetic outflow results in the release of acetylcholine (ACh) onto M3 muscarinic receptors. This results in the following effects:
- Acinar cells increase the secretion of saliva
- Duct cells increase HCO3– secretion
- Co-transmitters result in increased blood flow to the salivary glands
- Contraction of myoepithelium to increase the rate of expulsion of saliva
Overall, increased parasympathetic stimulation results in an increased flow of saliva that is more watery in composition.
Clinical Relevance – Sialorrhoea
Sialorrhea is drooling or excess saliva that cannot be controlled. There are two mechanisms by which this can occur:
- Lack of swallowing – resulting in saliva pooling in the mouth. This is typically due to neuromuscular dysfunction such as cerebral palsy, Parkinson’s disease or Motor Neurone Disease.
- Increased secretion of saliva – which is typically due to medication. It is often noted within the treatment of Alzheimer’s disease or myasthenia gravis, as treatment of both conditions involves the use of anti-cholinesterases.
The management depends on the cause but typically involves the treatment of any reversible factors. For example, drug regimes may need to be changed or even stopped depending on the severity. Behaviour modification may be necessary in terms of learning methods to help clear the pooled saliva.
In more severe cases, anticholinergic medication can be used to reduce the secretion of saliva – however, the use of this may be limited by side effects. Finally, if nothing else is successful; radiation, injection of botulinum toxin or surgery may be considered.